1. Field of the Invention
This invention relates to providing an ability to vary the data rate at which compressed video information is transmitted, and, more particularly, to varying such a rate in a manner providing the smooth display of video images.
2. Background Information
The playing of motion pictures and other forms of video is generally accomplished using a rather old technology, in which the primary storage media is video tape on which a television signal is recorded in an analog format. This type of storage is used in devices ranging from VHS.RTM. VCRs (Video Cassette Recorders) for the home market to the D1 VTRs (Video Tape Recorders), used by television studios and broadcasters, which cost $75,000 to $150,000. In the broadcasting industry, a particular limitation of such expensive equipment arises from the fact that it can only be used to perform one function at a time. Furthermore, the efficiency of the tape process is limited by the labor required to load and unload tapes, and by the accumulation of wear in tape transport units, tape heads, and in the tapes themselves. In particular, analog video tape is difficult to use when a tape has to be edited, as when two or more sources of programming, such as a motion picture and a number of commercials, are to be combined in sequence.
Video, or multimedia, data is generally delivered to end users either by broadcast industry methods or by computer industry methods. Broadcast industry methods generally include the storage of data in the form of analog or digitally recorded tape. Computer industry methods generally provide storage on disks, or on disks augmented by tape, with data being recorded in a compressed digital format.
An important example of such a compressed digital format is found in the MPEG video compression standards, which have been developed by the Motion Picture Experts Group to facilitate the recording of video data on CD-ROM disks and the playback of such data to either a standard television set or a computer system. Without compression, the demands of digital video in terms of video bandwidth and storage capacity are enormous. In an example described by Jan Ozer in Video Compression for Multimedia, AP Professional, p. 8, a typical uncompressed video file has a resolution of 320.times.240, providing 76,800 pixels in each frame. Thus, to capture each frame with a system using 24-bit color requires about 230 kB (killobytes) or 1.84 Mb (megabits). If this data is captured at a rate of 15 frames per second, which is half the standard rate at which television frame are transmitted, the bandwidth required is 3.456 MBs (megabytes per second) or 27.7 Mbs (megabits per second). However, a double speed CD-ROM drive transfers only about 300 kBs, less that a tenth of the bandwidth required.
As further described by Jan Ozer, ibid., pp. 364-365, a first MPEG standard, now known as MPEG-1, was introduced by the MPEG committee in 1991, with a video standard being built around the Standard Image Format (SIF) of 325.times.240 pixels at 30 frames per second, with a data rate of 1.5 Mbs (megabits per second). A second MPEG standard, known as MPEG-2, adopted in the spring of 1994, is a broadcast standard specifying playback at 60 frames per second, with data rates ranging from 4 to 16 Mbs. Even when the MPEG-1 standard is otherwise used, the data rate can be varied, with data being transmitted, for example, at 1.544 Mbs, 2.048 Mbs, and 3.0 Mbs. Lower data rates favor the use of a more simple systems for transmitting and decompressing the data, while higher data rates favor use of the system in connection with larger display areas, requiring more pixels of data, with motion picture sequences involving faster and more complex movement, and in applications requiring higher picture quality.
One application for video data compressed according to the MPEG standard, which is becoming increasingly important, is the use of a computer system as a video server providing movies in an MPEG format to a cable television system, which may extend, for example, into rooms throughout a hotel facility or into homes within a residential area. Each television set is connected to such a cable system through a "set-top" box, which may be placed atop the set. Each set-top box converts data received in the MPEG format into a format that can be received by a standard television set, such as the NTSC (National Television Standards Committee) format of North America, Central America, and Japan, the PAL (Phase Alternation Line) format of most of Europe, the Middle East, Africa, and South America, or the SECAM (Sequentiel Couleurs a Memoire) format of France, Russia, and parts of Africa.
One limitation on the use of MPEG data is the expense of its generation, or "capture." According to David Moskowitz and David Kerr, et al., in OS/2 Warp, Unleashed, Sams Publishing, pp. 752-753, the adapter hardware to capture MPEG images costs from $10,000 to $20,000, and service shops typically charge $100 per minute of video to generate MPEG data. This complexity in the generation of MPEG data implies that, while video segments recorded using the MPEG format can be readily joined with one another in an editing process, once a segment has been generated, its format cannot be readily changed, for example, for transmission at another data rate. Thus, a problem is developed whenever there is a need to join sequentially video segments which have been recorded using the MPEG format at differing data rates. That is, while a communication card for use in a computing system has a capability of transmitting MPEG data at various data rates, and while a set-top box receiving such data has a capability of receiving at several data rates, what is needed is a way to transmit segments at different rates sequentially in a manner which appears to be "seamless" or smooth to a person viewing the television monitor associated with the set top box.